Richard L. Caldwell

Formation evaluation by inspection with the borehole televiewer

The televiewer takes an oriented acoustic picture of the inside of the well bore in the form of a continuous well log. The resulting log is a representation of the well bore wall as if it were split vertically along magnetic north and laid out flat. Both induced and natural fractures are defined in remarkable detail. Well bores can be filled with any homogeneous liquid such as lease crude oil, water, or drilling mud. Resolution of the tool is sufficient to locate fractures 1/32 inch wide. Examples of natural fractures in extremely fractured formations, in shales, and also drilling‐induced fractures are discussed. Fracture orientation information useful for local and regional tectonic studies is presented.

CYCLIC ACTIVATION ANALYSIS.

Abstract Cyclic activation analysis is a technique which utilizes very efficiently a low-output, pulsed source of neutrons and short-lived radioactivities for analytical purposes. With this technique radioactivities having half lives of a few minutes to as short as a few milliseconds can readily be used as a basis for elemental analysis. This paper presents a mathematical analysis of cyclic activation, some experimental studies verifying the mathematical predictions, application of the technique to a specific analysis problem, and some suggested areas where cyclic activation could possibly be used to good advantage.

Dual-spaced neutron logging for porosity

Two-group neutron diffusion theory has been applied to two concentric cylindrical regions representing borehole and formation to show that the ratio of thermal neutron flux at two appropriately large distances along the axis is virtually identical to the epithermal flux ratio, and is a measure of a single epithermal parameter of the formation, viz., the slowing down length. The slowing down length is most strongly dependent on hydrogen content. It is shown that variations in salinity of the liquid in the formation or the borehole and uncertainties in borehole size have relatively little effect on the flux ratio. Experiments in a borehole model showed that the thermal flux ratio does not depend on eccentricity of location of the logging tool in the borehole. Fits of theory to experiment allowed determination of slowing down lengths for Pu-Be neutrons in salt water filled sandstone at two porosities for which experimental values have not previously been available. These investigations suggest that an improved porosity log can be obtained by use of 70- and 90-cm source-detector spacings with a neutron source of 5 X 10 8 neutrons/sec or greater. Proportional counters containing He 3 are recommended as detectors. Such a system employing flux ratios is expected to be insensitive to casing and cement in cased holes.

Acoustic well logging device for detecting shear and compressional waves

Novel acoustic logging tools are disclosed for logging unconsolidated formations. These tools include point force shear wave, and multidirectional compressional wave transmitters. A plurality of receivers with sensitivities matched to these transmitter types are provided for selectively gathering P wave and S wave information. The transmitters and receivers are located, sequenced and gated to ensure that high quality wave data is rapidly and reliably collected.

Low Voltage He3‐Filled Proportional Counter for Efficient Detection of Thermal and Epithermal Neutrons

Neutron counters using He3 gas at absolute pressures up to 10 atm have been constructed and tested. The relative detection efficiency for thermal, epicadmium, and fast neutrons was measured as a function of pressure from 1 to 10 atm, and compared with similar data from a boron trifluoride counter of the same size filled to a pressure of 65 cm of B10F3 gas. The ratio He3/BF3 increases approximately linearly with pressure for epicadmium and fast neutrons, reaching values of 26.6 and 21.6, respectively, at 10 atm He3 pressure. This ratio shows a saturation effect for thermal neutrons, becoming 4.45 at 10 atm He3 pressure. The gamma‐ray response has been found to be low with proper pulse shaping. High voltage plateau curves for 3‐ and 7‐atm counters are given and indicate operating regions at least 100 V wide at a counter high voltage of about 1200 V.

Uranium assay logging using a pulsed 14-Mev neutron source and detection of delayed fission neutrons

An in‐situ uranium assay logging system has been developed that measures directly and quantitatively the uranium concentration in the formation surrounding a drill hole. System operation is based on the DFN (delayed fission neutron) method which involves (1) bombarding a formation with short duration bursts of neutrons from a pulsed‐neutron generator to induced fission in any uranium present; (2) separating delayed fission neutrons, from source and prompt fission neutrons, by waiting a few milliseconds after each neutron burst before activating the neutron counter system; (3) deactivating the neutron counter system before the beginning of the next neutron burst; and (4) repeating this bombard‐wait‐count cycle a sufficient number of times to accumulate a statistically acceptable number of the delayed neutron counts. The DFN logging method has been used routinely in our field operations for several years, and it has successfully overcome one of the most perplexing problems encountered in uranium exploration...

ACTIVATION ANALYSIS IN PETROLEUM EXPLORATION RESEARCH

Abstract In many geological studies detection of the principal and minor elements in rocks is of interest. Neutron activation techniques are described for the rapid, quantitative determination of silicon, aluminium and magnesium, three of the most common elements in the rocks of the earths crust. The accuracy and reproducibility of the analyses and interferences from the other elements in sedimentary rocks are given. Neutrons for activation were produced with Van de Graaff positive-ion accelerators using the reaction deuterium-tritium to give 14-MeV neutrons and the reaction deuterium-beryllium (plus a water moderator) to give slow neutrons. The activation techniques were checked against standard samples previously analyzed by other methods and against known mixtures of pure elements. Typical results obtained in the activation analysis of approximately four hundred core samples are given. The core analyses are described in connection with correlation studies in a limestone formation, identification of dolomite in carbonate rocks, measurement of the shale content of carbonate rocks and aluminum and silicon content of siliceous rocks.

THE EFFECTS OF FLUID INVASION IN PULSED NEUTRON LOGGING

The weak dependence of the results of pulsed neutron logging on the presence of the oil well borehole suggests that thermal neutron die‐away measurements in an open hole may be capable of “seeing” beyond a limited amount of borehole fluid invasion. In order to obtain an estimate for the depth of investigation which can be expected from pulsed neutron logging when some invasion has occurred, multigroup neutron diffusion theory was used to calculate thermal neutron lifetimes for the open‐hole logging geometry. The transient, three‐group equations were solved with the aid of an IBM 7090 computer. Two cases of open‐hole fluid invasion were investigated: Fresh‐water filtrate invading a salt‐water‐saturated formation, and salt‐water filtrate invading a formation saturated with fresh water (or oil). In both cases the formation is assumed to be a 20 percent porosity sandstone and the fluid‐filled borehole is taken to be 20 cm in diameter. It is shown that for any given depth of borehole fluid invasion, up to the ...

GAMMA‐RAY SPECTROSCOPY IN WELL LOGGING

Thermal neutron capture gamma rays have been observed in boreholes drilled in shales, sandstones, and limestones. A capsuled source of neutrons and a scintillation crystal detector, connected through 5,000 ft of logging cable to a transistorized, multichannel, pulse‐height analyzer, were used. Resolved peaks were identified on the basis of the known energies of expected gamma rays and results obtained in models where conditions of porosity, casing, and fluid were controlled. To properly interpret borehole spectral data a system with good energy resolution and an accurate means of energy calibration are necessary. This is accomplished by using hydrogen and iron to give prominent gamma‐ray peaks at opposite ends of the energy range of interest. On field spectra, identification was made of gamma rays from chlorine, silicon, calcium, hydrogen, and iron. On the basis of chlorine gamma rays, salt water can be differentiated from oil or fresh water. Gamma rays from iron casing are an undesirable background and r...

GAMMA RADIATION FROM INELASTIC SCATTERING OF 14-Mev NEUTRONS BY THE COMMON EARTH ELEMENTS

Gamma rays in the energy range 2 to 11 Mev produced by inelastic scattering of 14-Mev neutrons by nine elements were measured at a mean angle of 90 deg . Excluding carbon and oxygen, the maximum energy gamma rays varied from about 8 Mev for phosphorus to about 10.5 Mev for magnesium and 1l Mev for silicon. Resolved gamma rays were observed from carbon (4.43 Mev), oxygen (6.1 and 7 Mev), silicon (1.78 Mev), aluminum (2.2 Mev), phosphorus (2.2 Mev), sulfur (2.2 Mev), and calcium (3.7 Mev). In the energy range 4 to 6 Mev there are indications of individual gamma rays in silicon; no resolved gamma peaks above 2 Mev were observed for iron and magnesium. Except for carbon and oxygen, the intensity of gamma rays decreases with increase in energy and varies from about 3 to 9 times higher at 2 to 3 Mev than at 5 to 6 Mev. Gamma production cross sections are given for each element, relative to the known cross section for carbon. The ratio of the integrated cross section for gamma-ray production above 2 Mev to the nonelastic neutron cross section varies from 0.59 for sulfur to 0.99 for iron. (auth)